Stacking machines are utilized primarily in the lumber industry to collect individual boards, although other products such as plywood, stakes, other wood products, drywall, etc. are also stacked, to facilitate shipping of the product to a distributor or retailer. Lumber, particularly boards or plywood used in the pallet industry, is typically produced in lengths between 2 feet and 6 feet, with thicknesses ranging from about ⅜ inch to 2.5 inches, and widths that range between 1.5 inches to about 48 inches. After the boards are sawn into the proper length, width and thickness, the lumber is generally gathered into layers, sometimes referred to courses, and then supplied to a stacker where the boards are placed into stacked packages that are typically between 12 and 128 or more layers high.
Lumber stackers are sturdy machines usually formed from steel with conveying devices that move the boards to a stacking area or station. Preferably, lumber stackers are operable at relatively high speeds so that the sawn lumber can be accumulated and then shipped to a remote point for sale or for further processing. Preferably, the stacking machine will be automated to receive the individual units of product and place the product into a stack to a desired size, and then discharge the stack to a remote location for shipment. Conventional stacking machines create a stack from the top of the accumulated stack, typically by utilizing a set of stacker arms to raise a course or layer of lumber. The stacker arms are then extended to a stacking station where the respective courses of lumber are collected into a stack. Once a course of lumber has been set on top of the accumulated stack, the stacker arms retreat to pick up the next course. This process is repeated until a stack of lumber of desired size has been created, whereupon the completed stack can then be bundled and shipped, or sent for further processing.
In addition, conventional stackers have not provided the ability to provide significant variation in the size of the stack in terms of the length of the individual boards being assembled into the stack. Although the production of individual boards of a smaller length is usually faster than longer boards, the conventional stacking machines have difficulty in accommodating the shorter boards and/or speeding up the stacking process. Stacking speeds of approximately 20 courses or layers of lumber per minute are often needed to match the speed of production of smaller boards.
The lumber stacker disclosed in U.S. Pat. No. 5,993,145, granted to Sidney Lunden on Nov. 30, 1999, provides for the building of the stack from the top of the stack by the placement of the individual course by stacker arms connected in an eccentric drive mechanism. Rocker arms provide stops for the building of the individual courses of lumber before the stacker arms place the accumulated course onto the top of the stack that is being built. In U.S. Pat. No. 7,651,314, granted on Jan. 26, 2010, to Gary Hogue, et al, the stacker arms are movable in a linear manner in conjunction with linear actuators that alternate the placement of different pairs of stacking arms to deliver a course of boards on top of the stack being formed.
The stacking mechanism disclosed in U.S. Pat. No. 8,613,585, issued to Jan Johannson, et al, on Dec. 24, 2013, is somewhat more complex than the stacking mechanism disclosed in the Hogue patent, but nevertheless builds the stack of lumber through the placement of the individual courses of lumber on top of the stack being built by alternating stacking arms. The use of alternating stacking arms increases the speed of operation of the stacking machine, but significant increases the manufacturing and operating cost of such stacking machines.
Accordingly, it would be desirable to provide a lumber stacking machine that can provide stacking speeds that will accommodate the production of different lengths of lumber at high operating speeds, while maintaining lower production costs for the stacking machine.